The topography of Vesta

The gravitational pull on Dawn is the cumulative effect of all the matter in Vesta. Gravity diminishes with distance, and the spacecraft is subjected to a changing force as the inhomogeneous protoplanet rotates and the ship revolves around it. When Dawn is closer to locations with greater density, it experiences a stronger tug and when it is near regions with less powerful gravity, the attraction is weaker. By carefully mapping the exquisitely small variations in the probe's orbital motion, navigators can calculate how the mass is distributed within Vesta. This has already enabled the discovery of a dense iron core, one of the reasons scientists believe it has a complex geological history more akin to planets than to typical asteroids.

The Vesta cartographic coordinate system used in the Gazetteer of Planetary Nomenclature and on this map has not been approved by the IAU Working Group on Cartographic Coordinates and Rotational Elements. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The orbit is calculated with astonishing accuracy using several methods, with the principal one being the measurement of the Doppler shift of Dawn's radio signal, in which the frequency changes as the spacecraft's speed changes. To map the complex shape of the gravity field, the team had wanted to accumulate a total of about 26 days worth of Doppler measurements at the Deep Space Network using the main antenna when it was pointed to Earth and one of the auxiliary antennas some other times.

Scientists are thrilling to the experience of turning Dawn's fantastic bounty of data into knowledge. As they discover and become more familiar with the features on what was so recently an entirely uncharted world, they are naming more and more of them. The growing list of landmark names approved by the International Astronomical Union is here.

Although Dawn is gradually receding from Vesta from May 1, many more observations are planned before it leaves for Ceres on August 26. Meanwhile, that still more distant world, another relict from the dawn of the solar system, waits patiently. Dawn and Vesta now are 2.5 AU from the sun, but Ceres is even more remote, and the ship will have a long journey to reach it in 2015. The craft has been in flight for more than four and a half years, so Earth has revolved around the sun more than four and a half times since it dispatched Dawn on its interplanetary adventure. The spacecraft itself has completed just over two heliocentric revolutions during that time (some of it while accompanying Vesta).

On April 30, Dawn was 210 kilometers (130 miles) from Vesta. It was also 3.47 AU (518 million kilometers or 322 million miles) from Earth, or 1,380 times as far as the moon and 3.44 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, took 58 minutes to make the round trip.

Source: 

• Dawn Journal, Marc D. Rayman, April 30, 2012